Method of making a capacitor utilizing bonded discrete polymeric film dielectrics

ABSTRACT

Capacitors employing discrete polymeric film dielectrics adhesively bonded to metal foil electrodes have been found to be effective up to and including high voltage applications of either A.C. or D.C.

[ Dec. 3, 1974 [56] References Cited UNITED STATES PATENTS METHOD OF MAKING A CAPACITOR UTILIZING BONDED DISCRETE "t" 71 .nd "d" "0 no" "0 mom m w a eie i hchf ISt euek SNRNP 50226 36666 9 9999 HHHHH 93385 84705 9 55 40600 3245 22 33 d n I .m d S C w c I n m mm d 0 C n C. E d L .e &n E m y 1 D B 0 l I M M a P L M .I. S n F m .m mh a I m M Wm t n u m .m 0 v a Pm A .l 5 3 7 7 [22] Filed: Sept. 14, 1973 [21] App]. No.: 397,240

. Primary Examiner-Carl E. Hall Related U.S'. Appllcation Data [60] Continuation of Ser. No. 212,806, Dec. 27, 1971,

Attorney, Agent, or Firm--Hoffmann Meyer and Hanson abandoned, which is a division of Ser. No. 54,187, July 13,1970, Pat. No. 3,649,892. ABSTRACT Capacitors employing discrete polymeric film dielectrics adhesively bonded to metal foil electrodes have been found to be effective up to and including high voltage applications of either AC. or DC.

10 7 60% 2 I32 71 l 3M3 @H% 2 /u 7 u .2 3.mA 9 n 2 5m 2 9 2" C St Um 1]] 2 8 555 Field of Search 7 Claims, 7 Drawing Figures II II I- II I1 1' II II I III III II VIII-III.IIIII-llllllll'nllllllll III II III II I. I. III Ill I. II II. I-

PATENTELBEC 3' .NNEH! HI NMM M METHOD OF MAKING A CAPACITOR UTILIZING BONDED DISCRETE POLYMERIC FILM DIELECTRICS This is a continuation of application Ser. No. 212,806, filed Dec. 27, 1971, now abandoned, which in turn is a division of application Ser. No. 54,187, filed July 13, 1970, now US. Pat. No. 3,649,892.

BACKGROUND High voltage capacitors of the prior art employ a liquid dielectric such as an oil with paper or paper and film between the electrode foils. Aside from the recognized problems with the liquid they generally require increased electrode spacing to withstand a given potential and some types can not be used for extended periods of DC. applications.

Known state of the art capacitors having discrete polymeric film dielectrics for high voltage applications have shortcomings with respect to arcing or corona discharge between the electrqdes either internally or at the electrode edges orboth. There are also problems with lead attachment, inductance, limited environmental characteristics, etc.

Polymeric film dielectric capacitors of the prior art have somewhat of an open structure, having some air space between the dielectric film and the foil electrodes. Also, void areas at the ends of the windings provide air paths between the electrode edges where corona or are discharge can occur. Furthermore,'these capacitors lack rigidity and strength of lead attachment to the foil electrodes.

OBJECTS It is an object of the present invention to provide film type capacitors and a method of making the same which have increased breakdown voltage.

It is another object of the present invention to provide capacitors and a method of making the same which reduces or eliminates arcing and corona discharge.

It is another object of the present invention to provide capacitors and a method of making the same which reduces the size of the device for a given rating.

It is another object of the present invention to pro vide capacitors and a method of making the same which will withstand high operating temperatures.

It is another object of the present invention to provide capacitors and a method of making the same which can operate on A.C. or DC.

Still another object is to provide a film dielectric capacitor having a highdegree of rigidity.

Another object is to provide a film type capacitor having strong'lead attachment to the foil electrodes.

Another object is to provide a film capacitor capable of withstanding high shock-and vibration.

Yet another object is to provide a wound film type capacitor having practically low inductance.

Another object is to provide a parallel plate or stacked assembly of foil electrodes and polymeric film dielectrics which retains its structure over a broad temperature range without the aid of an external retaining force or auxiliary structure.

Still another object is to provide a capacitor made of foil and polymeric film capable of operating up ,to 3,000 to 5,000 volts per mil of electrode spacing withi Other objects will be apparent from the following de scription and drawings.

The capacitor construction of this invention makes improved use of certain polymeric dielectric films, including the newer high dielectric strength films, by enabling them to function nearer their intrinsic voltage capability. This is accomplished by the use of selected polymeric bonding agents within and about the structure to bond the electrode foils to the dielectric film. Not only does this technique provide a highly rigid structure but mainly it provides for. the virtually complete exclusion of voids between the foil and film, and it fills in all voids areas about the outer extremities of the capacitor particularly between the edges of the extending dielectric films. This covers over the inset edges of the foil electrodes thus providing several times higher insulation against corona and arcing at this point than that provided by air in conventional film capacitor constructions. Additionally, the electrode edges not inset terminate and are exposed at the surface of the structure to which termination is made by a proprietary method as described in application Ser. No. 13,688, filed Feb. 24, 1970. Furthermore, it has been found that by the proper selection of bonding materials and methods of use the residual bonding agent remaining between the electrode foil and dielectric film does not contribute appreciably to reducing the capacitance.

This concept is particularly indicated for very small to moderate size devices especially where higher voltages of operation are required. Among other applications would include high energy storage, laser, missile, night vision, commutating and television circuitry.

In the drawings, FIGS. 1-4 are sectional views illustrating various stages of preparation of the capacitors of the present invention.

In FIG. 1 offset positioned metal foils l0 and 11 aRe shown. Assembled thereto in a manner to be hereinafter described is at least one polymeric film 20. Because of offset positioning of foils void spaces 12 occur near, the edge. At least one adhesive binder 14 is applied so as to substantially fill void spaces 12 as described in' greater detail hereinafter. Next the assembly is pressed, FIG. 2, preferably at elevated temperature. This decreases the thickness of polymeric adhesive layers 14 and forces the excess material 14 into voids 12 and to the outer extremities at 13. The surplus solidified binder 14 is removed as shown in FIG. 3. As shown in FIG. 4, the edges 16 of foils 10 and the edges 15 of foils 11 respectively are electrically connected together by applying terminating coatings 30 and 31, of metallic material, for example by spraying a nickel-aluminum composition as described in application Ser. No. 13,688, filed Feb. 24, 1970, assigned to the same assignee as the present application which is hereby incorporated into the present application by reference. Leads may then be applied to 30 and 31 to connect the capacitors into an electrical circuit. If desired for some applications the capacitors may be encapsulated in a known manner, preferably with a protective polymeric coating before or after attachment of leads.

FIG. 5 shows a wound construction of foil and film 50 having incorporated therein an adhesive bonding agent (not shown) by methods described hereinafter.

FIG. 6 shows the same construction after pressing,

out the use of an external protective environment. 7 and 31 as referred to hereinbefore.

In general, by the term "film materials is meant a flat section of polymeric material which is thin in relation to the length and breadth and which generally has a nominal thickness or gauge of not greater than about 0.010 inches (10 mils). Such films may be used with at 5 least one polymeric bonding agents to bond same to the electrodes.

A wide variety of polymeric materials may be utilized to produce these films. These polymeric films may be selected from among the following. One family of materials which is particularly suited to film production is olefin polymers. Included in this term are polymers which polymerize off the basic double bond monomer illustrated below.

polymers also, according to well-known methods,

which have varying properties. It is also possible, as is wellknown, to cross-link many of these materials, eg

l q ute nss i n eme??? Preq 15 Representative examples are the homologous series 3 of poly(ethylene), poly(propylene), (poly( 1- octene), etc.; structural isomers such as poly(isobutone) and poly(butene-1 and a wide variety of copolymers thereof. Other'examples are thevinyl, vinylidene,

2O vinylene, acrylic, methacrylic, and cyanoacrylic series (see below) and copolymers thereof.

Thus if R1, R and R are hydrogen R4 may be any of the groups known to those skilled in the arts; e.g. Cll Cll -ll; alkyl, aryl nlkaryl, aralkyl and sub- 2 l n stituted allcyl, airy] alkaryl, nralkyl I R groups; halides hydroxyl; ether groups;

carboxyl; ester groups; cyano; heterocyclic (vinyl) groups; etc. i

If R and R are hydrogen for example R3 and R4 -Cl lCll 3, -C l-l 400m CN,'

cu 0H N I (pyrrol done) C Cll In the case of vinylidene monomers, some of the above R groups will not serve as well as others clue to steric hindrance and/or electronic considerations Some examples of the subgroups within the acrylic "family" are:

2 R H acrylic Cll methzlcrylic -CN cyanoacrylic acrylic, etc. -Cl chloroacrylic Others are pdsibiea iswfl knowntothose skilled in the art. R e.g. may be COOR'; CONR' where R' is H; alkyl, aryl, alkaryl, aralkyl or'substituted alkyl, alkaryl, aralkyl and aryl groups.

' 6 the like; tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene, and the like, modified polymers such as polyvinyl-formal, -acetal, -butyral,' and the like; as

water ess-calledlisrzqntanseysfres.. sa P ly- Vinylene monomers'differstructurally from the vinylidene in that the two substituents are attached to adjacent carbon atoms rather than the same carbon different than Z.

CH I, j n Y Z vinylene atom CH CH poly(vinylene carbonate) c -cu be poly(acenaphthylene) cyanostyrenes, nitrostyrenes, and the like; vinylbiphenyls; vinylnaphthalenes, vinylterphenyls; vinylfluorenes; 9-methylenefluorene; vinylphenanthranes; vinylpyrenes; vinyldibenzofuranes; vinylcarbazoles; vinylphenoxathiins; and the wide variety of other similar monomers well known to those skilled in the art. Still other examples are polymers and copolymers of butadi- E p 'ne sblsr ne 4rm thyl-psntsnsrl 95 polflethyleneterephtlialate) Substituent Y may be the same or Some examples of suitable groups were given above as R Again, not all groups will permit polymerization, as is well known to those skilled in the art. groups may represent one substituent as is shown for vinylene carbonate and acenaphthylene.

Also, the Y and Z mers of poly(paraxylylene), its derivatives and the like. Of these polymers included in the double bond poly merization, unorientated, polystyrene, halogenated monomers such as polyvinyl chloride, polyvinyl fluoride, combination polymers such as vinyl chloride, vinyl acetate, polyvinyladine chloride, polyvinyl acetate, polyvinyl alcohol are preferred.

A second group of polymeric materials from which films may be prepared is condensation polymers. Condensation polymers are prepared by wellknown techniques, and include polyesters; polyamides; polyesteramides, polyurethanes; polycarbonates; condensations of formaldehyde with phenol, resourcinol, melamine,

,urea, and the like; phenoxy resins; polyphenylene,

polyphenyleneoxide; polyphenylenesulfide; polydiphenyleneoxide; polysulfones and copolymers thereof,

and the like; and copolymers with each of the above with each other. Some example s are depictedbeloyv,

an aromatic polyester Cll 0 i a Ii t 1h in oco lc- Q] J Ltco O n J r n Cll polycarbonate of Y polycarbonate of 2 2 bis (4-hydroxy phenyl) propane 2 ,2 bix (4-hyclroxy phenyl) norbor- (Polycarbonate I) T nane Y a (Polycarbonate II) 3 g o@-c OCH CH CH @S- i--.

' 2 2 n v j n on I on f phenoxy resin poly(phenylenesulfide) poly ates (phenyl) CH o v i ll H CH 0 V a polysulfone polyetther copolymer j Esters are formed by the action of polybasic acids or 3 5 teristic structure CONH among hydrocarbon residues their anhydrides with polyhydric alcohols. The polyes- (CH )n. This includes the two main types, polyamides ters may be utilized in e ther the orientated Or obtained by condensation of diamines with dicarboxic ated o it sg if y are orientatedacids and polyamides obtained by condensation of one Particularly advantageous y of polyesters is the amino acids. Exemplary film materials include the res cellulose esters, exemplary materials in this category i f fi type nylons OC(CH2)11 CQNH(CH2) include cellulose, cellulose acetate, cellulose acetate- NHCQCHQ" 0NH c NH and m are i buterate, ethyl cellulose, methyl cellulose, cellophane gers, ll n t ab ve 10,

with or without a polymer coating, cellulose alone has A oth r condensation polymer' is polyurethane,

the formula [(C H O )OH (n). Additionally, celluwhich is formed according to the reaction, of diisocyalose ethers may be formed. They generally have the fornatewith two or more hydroxyl groups. OH-ROC- mula [(C H O O-COR) where R is methyl, ONH--R'-NHCOOROCONH-RNHCOO.

ethyl, benzyl or other desired radicals. Many of the newer polymers, available commercially Cellulosic polymers should be recommended with or on a developmental scale, possess heterocyclic linkdiscretion. They have high dielectric constants but, in ages in the chain. MOst of these have excellent electrigeneral, high-dissipation factors and limited tempera- Cal Properties and extremely high temperature Capabilture capabilities, depending upon substituents. Some of itie e ype is a p yim his c a zed by the the commercially available derivatives of value, when pr sence of the grouping. used within their limitations are: the cellulose esters, r C such as -acetate and -buterate; cellulose ethers, such as methyland ethyl-cellulose; hydroxyethyl-, and carboxymethyl-cellulose. I N

Another type of condensation polymer which may be I used is polycarbonate. Polycarbonates are esters of di- C phenylolpropane. Both cast and extruded polycarbon- H ates films may beused. Detailed information regarding particular polycarbonates and their methods of prepa- O ration may be obtained from the book Chemistry and It can be supplied as a polyamic resin solution, which Physics of Polycarbonates by H. Schnell (1964 John upon removal of solvent and heating converts to a poly- Wiley & Sons Inc.) wh ch iS hereby incorporated into imide by the elimination of water. This is shown below the present application specification by reference. a a preparation from pyromellitic dianhydride and Additional condensation polymers which may be v bis(4-aminophenyl) ether, with subsequent conversion used are the polyamides. Polyarnides have the characto a polyimide.

0 o H N- O 0 O 1H C C Q I 2 l H t H w a PMDA 1 BAPE II II C 'C X o HOOC COOH H H n "polyamic acid" Heat o l on H 0 ll ll Cv C\ C c t I II II o o r n polyimide Representative examples are given below A wide variety of properties can be obtained by varying S Properties can be obtained by 9 the dianhydride, the diamine, or the processing condiw at slml!ar.polymers Such as polyamlqedmldes. an

' I 3 polyester-lmldes. Here too, propertles Wlll vary wldely depending upon the amine and anhydride employed.

a polyamide-imide a polyester-imide polymer chain is of indefinite length, containing the siderably lower than 100; as is well known to those units parenthesized. However, the values of n, m, and skilled in the arts. p in the formulas are generally at least about 100 more l leterocyclic rings other than imide are incorporated preferred at least about-1000, still more preferred at 5 into polymers, again usually to give hightemperature least about 5,000 and most preferred at least about resistance and good electrical properties. Examples are 10,000. The usable values will vary considerably bethe polyimidazoles, polythiazoles, polyoxazoles, po-

tween different types of polymers. In general n, m, and lyoxadiazoles, polythiadiazoles, polytriazoles, polytetp should not be so high as to cause processing difiiculraazopyrenes, polyquinoxalines, polyimidazopyrroties, 80 low as t0 result in Structural w knggg 10 lones and the like, including polymer chains containing Moreover, the values for resinous materials such as c er: two or more f th above linkages.

H'N NH I C C i N N n a poly(benzimidazole) g n a poly(benzothia2ole t N N a a poly(benzoxazole) I N N N v N v C C O C\ /C.

v 0 Q S i a 'poly(oxadiazole) a poly(thiadiazole) N N i a C /C a poly(triazole) regarding polymeric film materials can be found in Kirk-Othmer Encyclopedia of Chemical Technology 2nd Ed. Vol. 9, l966 pp. 221-244 and references cited therein. Additional information on the polymers per se, their structure, properties, methods of preparing-etc. may be found in Mayofis Plastics Insulating Materials,

' lliffe Books, Ltd., London, D. Van Nostrand Co. Inc. Princeton, N.J., 1966. Both of these references are hereby incorporated into the present application by reference. v

It should be noted at this point that all the above polymers may not be suitable for all dielectric film uses,

Dielectric inch film) 7,000 VAC at 25C. 6,000 VAC at 150C i Volume Resistivityvs. Temperature l X ohm cm at 25C, 1 X 10"+at 150C Insulation Resistance vs. Temperature 1 X10 megohms X uF at 75C 1 X 10 megohms X uF at 125C 1 X 10 megohms X uF at 150C Since the dielectric constant curve for polyimide film is rather uniform from about60C to 220C this makes for good capacitance maintenance in this temperature range, however, in going from. 60C to about 20C the K rises from about 3.05 to about 3.4-3.5. In the case but each is suitable within its limitations. For example,

.low dielectric constants. Thus, the use of any polymer polymeric film materials have been found to be applicable in producing high voltage film capacitors either in a stacked or wound configurationf Polypyromellitimide (polyimide) (PI) film, and polyethylene'terephthalate (PETI-I) have been found to be highlyuseful for certain applications. Polyethylene terephthalate does not have the high temperature properties as the polyimide but it does have dielectric strength equal to or slightly greater than the polyimide. Also this is a much lower cost dielectirc. However, at the high temperatures such as 150-200C the dielectric strength is less than polyimide. The following data on electrical properties for both these dielectric materials are listed below for comparative purposes. (polyethyleneterephthalate) Dielectric Constant 'vs; Temperature (60 Hz) 3.16 at 25C, 3.7 at 150C Dielectric Constant vs. Frequency (25C) 3.12 at 11414,, 2.98 at lMl-l'z a Dissipation Factor vs. Temperature (60 Hz) 0.0021 at 25C, 0.0064 at 150C Dissipation Factor vs. Frequency.,(25C) 0.0047 at lKl-lz, 0.016 at lMI-Iz Dielectric Strength vs. Temperature (60 Hz) (A.C. volts per mil on 0.001 inch film) 7,000 at 25C, 5,500 at 150C Volume Resistivity vs. Temperature Y 1 X. 10" at 25C, 1 X .10 at 150C ohm cm. Insulation Resistance vs. Temperature l0 megohms X uF at 75C (0.001 inch film) 800-1 ,000 megohms X uF at 130C 80-l00 megohms XuF at 150C (polyimide) j Dielectric Constant vs. Temperature (100 Hz) 3.5 at 25C, 3.1 at 50C, 3.0 at 150C Dielectric Constant vs. Frequency l00--l00,000 I-lz practically no change v Dissipation Factor vs. Temperature (100 Hz) 0.002 at R.T., same at 150C. 0.0045 at 220C Dissipation Factor vs. Frequency (25C) 0.002 at 100' Hz, 0.008at lQ 1-Iz of PETH, the K rises from about 3.56 at. 25C to about 3.7 at 150C.

1 Therefore to obtain a more uniform capacitance over a broad temperature range a combination of dielectrics such as PETH and P1 or others may be used such as a film of each of equal thickness or of unequal thickness ,to achieve certain desired electrical characteristics.

Since the dielectric strengths of both materials are about equal andth'ey are of about the same dielectric constant, this should be feasible,not only to obtain a more uniform capacitance over the temperature range up to about 125-150C but also they compensate for anamolous characteristics of each other in other respects. a

In laminated capacitor testswith Pl film where the objective was to make units which would safely operate at 10,000 V, there was not a substantial increase in breakdown voltage in going from 0.002 inch film to 0.003 inch film. This is consistent with the general rule that dielectric strength is much greater on thin films than on thick films on a per mil basis. In view of this it should be preferable from the dielectric withstand voltage standpoint to use multiple films. The use of multiple thin films may be of the same material such as PETI-I or P1 or others. The multiple thin 'films should be bonded together throughout or bonded together at the outer edges of the dielectric.

In addition to PETI-I and PI films as dielectric there are other film dielectrics which may be applicable,

however their dielectric strengths are somewhat lower. The following are also applicable for some uses in that they can be bonded to the electrode foils by applicable bonding materials. These would include: (1) polysulfo'n'e, (2) polycarbonate, etc.

Still a third group of film dielectric materials having moderate to good dielectric strengthand having otherwise good electrical properties are preferred for some applications. Some of these are low costfilms. However, they are not very amenable to bonding to the tric film to the foil electrodes and to fill in between the edges of the dielectric and cover or embed the inset foil edges, a wide variety of materials are applicable. The followingcharacteristics of the'bonding agents are desirable. (1) Fill substantially all void spaces between the film dielectric and the electrode foils to avoid corona discharge at the higher voltages, (2) to provide for a structurally rigid device to better withstand severe environmental conditions and (3) to afford means for a, strong lead attachment in a manner which does not improvide for a non-inductive capacitor device even though the structure may be made by winding the elec'- trode foil and film dielectric members together, concentrically, which usually, by the. prior art methods, re-

sults in a capacitorhaving various degrees of inductance.

of the following characteristics: a. good dielectric strength, but not necessarily high, b. good adhesion or bonding property to the dielectric film and to the metal foil electrodes,

0. moderate to 250 C, I d. not liberate substantial amounts of water or volatile gases during processing or curing, e. have sufficiently good rheological properties that under moderate mechanical pressure and heat it will be sufficiently. removed from between the dielectric film and the electrode foil that the resulting electrical properties of the dielectric will not be appreciably affected adversely,

Preferably, the bonding agent should have most or all high temperature capability, 100C to I f. should have sufficiently low viscosity and long pot I life that wound structures can be vacuum impregnated to fill in spaces between the dielectric film and electrode foil at least part way into the structure in order to provide rigidity to the foil and film edges and cover and fill in-the void area left by the opposite inset foil,

g. upon curing or setting to a solid, the bonding agent must not be too brittle otherwise the structure may shock.

h. lends itself to application to either the foil or film or both while in a flowable or liquid condition to the desired thickness, then converted to a low fusing solid film, or B stage, which can later respond to mechanical pressure and heat to force the excess material from between the dielectric films and electrode foils and said excess material filling in voids areas or spaces particularly about the electrode and film edges.

I Among the types of or specific materials which are known to have adhesive bonding properties to metal and to prevent arcing or corona between the foil edges,

form cracks resulting from mechanical and thermal foil electrodes and many dielectric film materials and while still in themonomeric and semi-polymeric stage.

The list of applicable materials is by way of example, I

only and it is obvious that new materials are under cur like, -45.

rent development and new ones will be produced in the future. Perhaps virtually all of the bonding materials listed above are applicablewhere the requirements of operating temperature,.voltage, and other electrical characteristics are not severe. However, for high volt-' age operation, such as 500'volts to upwards of 25,000 volts, otherwise good "electrical properties, broad operating temperature range etc. the choice of bonding agent would necessarily be limited to a fewer number of materials. As examples, two quite useful material groups, the polysiloxanes and the epoxies are treated in some detail to convey their versatile nature.

.Polysiloxanes are very suitable to use as adhesive bonding agents. They are commercially available as liquids, solids, gels and solutions of solid or semi-solid resins. Their preparation and use have been the subject of much patent and other literature, as has their curing agents and processes. One common method of preparation consists of condensing together one or more hydrolyzable silane monomers to give a structure simply and generally depicted below,

where the R-groups maybe varied very widely by proper selection of the silane monomers, e.g.,

(1) alkyl groups, straight'chain or branched, particularly the lower alkyls I (-2) alkenyl groups; e.g., vinyl or allyl, etc.

(3) aryls, aralkyl or alkaryl groups; e.g. phenyl, tolyl, benzyl, biphenyl, etc.

(4) substituted alkyl;- such as halides, hydroxyl, ethers, carboxyl, esters, cyano, nitro, sulfate andv the e.g., "-CH CH CI, Cl-l CH CH CN, CH CH CF etc.

(5) substituted aryl; such as halides, hydroxyl','ethers,

carboxyl, esters, cyano, nitro, sulfate and the like, e.g., C5H4F, C8H4OCH3, CaHaC] N03, etc.

(6) silane hydrogen; i.e., l-l (7) hydroxyl groups; i.e., *Ol-l (8) ether group OR where R is e.g., alkyl (8) ether group OR where Ris, e.g., alkyl A balance of variably predictable properties is obtained by varying these R-groups according to known procedures. The groups in (l), (2) and (3) are essentially non-polar and result in polymers having low dielectric constants (K=2,-3, e.g.) and low loss factors (DF= 0.00l, e.g.). Hence, they do not contribute markedly to any special or abnormal electrical effects, but they may be varied to give different handling characteristics in the fusible state, and physical properties in the curedstate.

Of particular suitability to use in this application are the polysiloxanes commercially available as coating and/or laminating resins. In these, some silanol groups are present on the chain, which may be represented very'generally by,

R R I I Si-O Si- I I R 0H tion, into an insoluble, infusable and adherent resin 120 mass.

' The ratio of R group to Si atoms (R/Si) is used to describe in a very average manner, the degree of branching of the polymer and the hydroxyl content, both gen- I erally being higher as the R/Si decreases. Suitable R/Si ratios run from 1 to under 3. Preferred resins have a R/Si ratio'of about 1.3 to about 1.9, and the preferred R groups are combinations of methyl and phenyl with a phenyl/methyl ratio of 0.1 to 0.9. A resin found to be very suitable has an R/Si of about 1.4 and a phenyl/- methyl of about 0.3. The fully cured resins have excellent electrical and thermal properties, and are especially valuable for use as bonding agents between electrode foil and film dielectric.

In addition to the above many polymers are modified with siloxane resins or compounds to lend increased moisture resistance and, often, increased temperatures capability. This is usually accomplished by reacting a etc. Thus, me'r'iiivanabie, for example, siloxaneepoxy resins, siloxane-alkyls, siloxanephenolics and thelike, the enhanced properties of which may render the base polymer more readily usable as a bonding resin CH CH CH l 2 One very important type of resin is the silicone potting and encapsulating resins. In one reaction the addition of silane hydrogen on one chain to a vinyl group on another, e.g.

R I I Si-0 Si-O I I H CH I C (H CH il 2 -Si-O- -Si-O All of the epoxy resin systems available today are useful adhesive bonding agents to a greater or lesser degree. They are usually discussed in terms of( l the resin and .combinations thereof, (2) the hardeners or catalysts,

(3) activators which may promote the resin-hardener reaction, and (4) modifiers which change the electrical and/or physical properties of the cured system.

Some of the more common resins, thoroughly discussed in patent and other literature, are

(l) diglycidyl ethers of Bisphenol A (DGEBA);

' (2) polyglycidyl ethers of phenolic resins (PGEPR);

(3) various compounds usually termed cycloaliphatic oxirane resins, e.g., vinyl cyclohexanediepoxide (VCHDE) and his (3, 4 epoxy--methyl cyclohexylmethyl) adipate (BEMCA);

(4) epoxidized unsaturated polymers, e.g. epoxidized butadienestyrene copolymers (EBSCP);

(5) epoxidized natural products e.g. epoxidized soybean oil (ESBO). They are all characterized by having more than one oxirane rings present. Simplified representative structures are given een IDGEBP PGEPR- where x is about 2 to 20 EBSCP cn cn------cn CH2 cn cn O /CH \ICH BEMCA H CO C-(CH CH CHCH -CHCHCH CHCll-CH CH ll llCO C-(Cll -CllCl'[-CH -Cll-Cll-(Cl'l --Cl[ 2 '7 a .2 4 3 I 0 g 0 ll C I R H200- C (9H ll Cll H lll ESBO Hm v g r J, With regard to the radicals, i,e., R, R, R R R and l I general, hardeners are used at or near the stoichio- R groups defined in this application, the aliphatic, cycloaliphatic alkyl, alkenyl, aryLaralkyl, alkaryl, aromatic, substitutedalkly, substituted aryl, substituted alkaryl, substituted aralkyl, groups preferably contain not more than about 20.carbon atoms, more preferably not morethan about l2-carbon atoms, most preferred not more than about eight carbon atoms. Further, the aliphatic, alkyl, alkenyl, and substituted alkyl groups most preferably contain not more than about six carbon atoms.

Epoxy resins react to give cured systems with most,

compounds containing a plurality of active hydrogen groups, such as amines, carboxyls, carbinols, phenols, mercaptans, and the like. They become chemically pentamine; aromatic aminessuch as metaphenylene diamine andmethylenedianiline. g

(2) anhydrides, e.g. phthalic-, maleic-, and methyl-3 metric ratio of active hydrogen to oxirane ring. Epoxy catalysts are usually differentiated from the hardeners given above, in that the catalysts promote the epoxideepoxide reaction without necessarily entering the polymer chain themselves; and are used in lesser amounts,

bound. into the crosslinked resin structure. Of the more- 6-endomethylene-tetrahydrophthalic anhydride; pyromellitic-, cyclopentaneand dianhydrides; dodecenylsuccinic polyazeleicpolyanhydride, and the like;

(3) flexibilizing hardeners, such as polyamides (Versamids), polysulfides (Thiokols), dimer and trimer acidsgthe last twoanhy'drides mentioned above, and hsli a benzophenoneanhydride;

e.g., perhaps 0.1-5 percent by weight. The more-common are Lewis acids and bases, and adducts thereof, e.g. borontrifluoride and its monoethylamine adduct,

and tertiary amines such as piperidine and 2, 4, 6-.

tris(dimethylolamino), phenol. These catalysts often provide latent curing systems; eg stable at roomjtemperature, but active hot. 1

Activators may be used to promote the reactlon between the epoxy resin and the hardener, most commonly to speed the'reaction between glycidyl epoxy groups and anhydrides. Lewis bases, such as those mentioned above or benzyldimethyl amine, are commonly used. Phenolic-moieties often enhance the somewhat salt of a weak acid, such as cyclohexanecarboxylic, in

order toobtain a reasonable pot life.

Not only the properties required in' the capacitor dictate or indicate the bonding agent to be employed but also the physical construction of the capacitor will influence the choice of material. As an example, a small is a factor in the choice of material.

The various bonding materials also have different physical properties before incorporating in the device, therefore they require different methods or techniques in their use. The following describes the various physical forms and how these relate to the methods by which.

be included both thermoplastic and thermosetting bonding materials. In general, solutions of the materials would be applied to either the electrode foil or the dielectric film or both, then the solvent evaporated to. leave a sufficient thickness of the coating on the member to later serve as the bonding agent after assembly into a stacked or wound capacitor structure then subjected to mechanical pressure and heat. With either thermoplastic or thermosetting materials, solvent removal must be rather complete to preclude its volitilization during the application of heat-and pressure causing voids in the structure. In the case of thermosetting resins and materials the solvent removal must be at sufficiently lowftemperature that the material will not be converted to the cured or infusible condition.

Examples of thermoplastic materials in this category are as follows:

a. Polyphenylenesulfide, a high temperature material All these materials are soluble in common organic solvents from which they can be applied to foil or films,

melting at about 290C. PPS is extremely limited in solubility, the best solvent being trichlorobiphenyl maintained at 200C or higher. The electrode or film member is coated with .the PPS by dipping in the hot solution and withdrawing with subsequent heating in air to about 300C to 340C to volatilize away the chlorina-' ted biphenyl at which temperature the PPS fuses to give a thin coating. Coating thickness can be increasedby repeating the operation several times since the previously applied fuse coatings are no longer soluble in the solvent. Although this is a very good bonding agent in many respects its use is limited to the very high temperature film dielectric materials such as-polyimideand polyamideirnide which are infusible and thus will withstand the temperature required to fuse the polyphenylenesulfide. Examples of other high temperature thermoplastic bonding would be b. Polysulfones which are soluble in a number of solvents such as N-methyl-p'yrrolidone, m-cresol N, N dimethylacetamide, dimethyl sulfoxide etc.'and

c. Polyethylene terephthalate'which is soluble in mcresol, trifluoroaeetic acid, o-chlorophenol, etc. Due to their high fusion points, about 300C, thesematerials are also limited in their use to high temperature film dielectrics such as polyimide.

Examples of low fusion temperature thermoplastics would include I a d. Cellulose nitrate,

e. Cellulose acetate,

f. Cellulose acetate butyrate,

g. Ethyl cellulose,

h. Vinyl chloride,

i. Vinyl acetate, 7

j. Polyisobutylene', etc.

erties.

then dried resulting in coatings which are fusible under the agencies of heat and mechanical pressure.

Examples of thermosetting materials which can be applied to foil electrodes or film dielectrics from their solutions would include:

i (a) Addition type polyimide for example in dimethylformamide. This type of polyimide releases virtually no gaseous reaction products during curing in contrast to the usual condensation type polyimide which releases much water vapor during curing resulting in severe void formation. In use, the addition type material is applied to the foil or film then subjected to an initial temperature of l50C-200C for 3-6 minutes to remove the solvent, then to 200C to 260C for 3-6 minutes to imidize the materiahln this form it is a fusible solid coating which responds to fusion and flow under mechanical pressure and heat at a temperature of 300C to 350C. Due to this high temperature the material can only be used with very high temperature film dielectrics such as polyimide and'polyarnideimide. its use however makes possible an all-polyimide construction which is polyimide has outstanding thermal and electrical prop- (b) Other applicable thermosetting type materials arethe siloxane coating resinsvThese may be applied from toluol solutions and the solvent removed'by heating at C to C for about 10-25 minutes. This results in a fusible coating which can be cured or polymerized under A mechanical pressure and heat of l50C-200C for periods of 1% to 15 hours. These resins rep resent intermediate temperature cures and thereby permits their use with other films in addition to the polyimide. These would include the polysulfones, v

All SFfis materialsare aromatic s olvents such as benzol, toluol, xylol, etc. from which coatings; can be produced. These solvents may affect certain dielectric films however in such cases this can be avoided by applying the material 'to the electrode foil.

2. Fusible tape: Of considerable applicability, are certain fusible films speciallyproduced for the bonding of metal to metal and metal to other materials. These tapes are based on thermosetting materials and therefore are infusible after curing and thus offer moderately high use temperatures. Another attribute is the absence of solvents and some do not emit gaseous products during the curing operation-Although these tapes can be used in wound capacitor constructions by winding in one of these members between each foil and dielectric film followed by pressing the winding between heated platens to fuse the bonding film to cause it to bond the dielectric film and foil together and to force out excess material, they are perhaps better indicated. for use in stacked or laminated constructions of small size. Here the bonding tape would be precut to the proper size and a piece usedin the layup between. each electrode foil and dielectric film. Pressing between heated platen causes the bonding tape to fuse and bond the dielectric film and foil together and to force out excess material to fill in between the edges of the films and to cover the edges of the electrodes.

that commonly used in making certain types of electrolytic capacitors. In the practice of this method the dielectric. films or electrode foils orboth are pulled One material available in tape form is based on 'a' some volatile by-products during cure thus leaving slight void spaces and thereby restricting their use.

Another type bonding material is elastomer-resin. This is superior to the elastomer-phenolic in that tensile shear strength of as much as about 5,000 psi is obtained, and no volatile by-productsare given off during cure. Additional advantages are higher service temperatures, about 125C, and lower pressures required, 50

psi to 200 psi to produce the bond. Although cure time LII These materials can be used with such dielectric film as polyimide, polyethylene terephthalate, polysulfone, polycarbonate and others if they are not adversely affected by the requiredcuring temperatures.

3. Solventless liquid bonding agents.

In this category are certain materials which are highly useful as bonding agents. They differ from the previous materials. mainly in that they are liquids but they contain substantially no volatile solvent. They are incorporated into capacitor constructions in their liquid condition without diluting with a solvent. As a group they convert to solids by use of curing agents with or without the application of heat. Another characteristic is that they do ,not liberate substantial amounts of gaseous byproducts during cure. Although these materials, as a group, contain no solvents may have sufficiently low viscosity that they have sufficiently good rheological behavior that only modest mechanical pressure need be applied to remove excess material from between the capacitor'elements of either stacked or wound construction even in relatively large sections.

The use of these materials in the assembly of capaci tors requires somewhat different techniques than the previously mentioned materials, primarily because they are liquids.

In making small stacked units starting with pre-cut foil electrodes and dielectric film, one or both are througha small container of the liquid resin material-as theunits are wound. Upon emerging, the members are brought between doctor blade scrapers orelastomeric rolls adjusted to leave the properamount of material on the members. By a similar process the film or foil or both may be allowed to contact transfer rolls which have a film'of the liquid resin material on the surface, thus sufficient material is transferred to the foil or film. By either of these or other methods, 'the resulting wound unit contains sufficient liquid material between the members that upon pressing between heated platens the excess is forced out from between the foil and film to fill in between the adjacent film edges and to cover the edges of the inset foil electrodes. This operation also converts the liquid to a solid which is bondedto the capacitor elements and to occupy substantially all void spaces within the structure.

Since, the materials in thiscategory contain no solventsand practically no volatile materials they are adaptable to vacuum impregnation of-dry wound units. This is feasible also because they are relatively low in viscosity, Although some compositions have viscosities ranging up to over 25,000 centipoises, the range for the low viscosity compositions is about 3,000-6-,500 centipoises. Although vacuum impregnation of woundunits is not likely-to be as thorough as by wet winding, it does offer some advantages, particularly in that drywinding of units is more rapid. After vacuum impregnation, the units are drained thenpressed between heated platens as described previously. I

These liquidresins are generally two-part systems, having a hardening or curing agent present during use, consequently they'generally have a limited pot life." Nevertheless, by the proper choice of curing agent or hardening agent and by use of slightly reduced temperature it'is possible to extend the pot life to'as long as 8-48 hours. I a. Solventless silicone potting and encapsulating res- These materials have very good electrical and thermal properties. They are quite adaptable to this-application by virtue of having the above properties along with good rheological characteristics.

They are, however, somewhat lacking in ri'gidity in the cured condition which sometimes may limit their use to certain types of constructions, particularly to small windings where slight flexibility can be tolerated and a high bond strength is not necessary.

Although similar products are available, from various manufacturers, one specific example representative of this group is Sylgard 182, a siloxane resin product of the. Dow Corning Corp. Although higher -.viscosity grades are available, this one has. a relatively low viscosity of 4,000-6,500 centipoises at room temperature. This material cures for example in 4 hours at C, 1 hourat C or l5 'minutes at, C. I

btEpoxy Resins. Although not havingjthe bestp'roperties in all respects, epoxy resins are one of the preferred materials for the practice of this invention. The broad range of compositions available provides for a high degree of versatility in their use.'l'n addition-to high bond strength and good electrical properties, com positions are'available which have relatively low-viscosities, in the range of about 3,0005,000 centipoises, and moderately high operating temperatures such as 150C and higher. Furthermore, they are versatile with respect to pot life and curing temperature and times.

EXAMPLE 1 Voltage breakdown tests were made on four aswound capacitors of approximately 0.02 uF rating and measuring about 2 inch long X 11/32 inch diameter. These were wound with two layers of 0.0005 inch thick polyimide film between 0.00025 inch aluminum foil electrodes. One edge of each foil extended out of one end of the winding while the other edge was positioned /a-3/l6 inch below the edges of the dielectric film at the opposite end to provide ample distance between the foil edges on each end. These units had breakdown voltages as follows:

1. 3500 volts D. C. 2 3000 volts D. C.

3700 volts D. C. 3500 volts D. C.

Breakdown occurredat the foil edges of all units.

Eight capacitors identical to' the above except the foil extensions where cut off then the units were vacuum Breakdown results were as follows:

1. 6000 VDC 5. 5000 VDC 2. 6500 VDC 6. 6000 VDC 3. 8000 VDC 7. 8000 VDC 4. 9000 VDC 8. 8000 VDC These values are substantially higher than those of the un-impregnated units and are in the range of the breakdown values expected of the film dielectric. There was no indication of breakdown at the foil edges.

Some of these units were sectioned and found to be rigid throughout showing excellent resin penetration.

EXAMPLE 11 Three capacitor windings identical to those in Example 1 above were first measured for capacitance and DE. then vacuum impregnated in the same epoxy resin composition for 45 minutes, then removed and pressed individually between heated platens for about l /2 hours at 550 lbs. during which the temperature was raised to about 175C to cure the resin. After removal from the press the excess hardened resin was removed from each end to expose the edges of electrode foils for electrical contact for measurement. One object of this test was to determine the effect of this treatment on capacitance and breakdown voltage.- I

lNlTlAL MEASUREMENTS Capacitance (uF) Dissipation Factor (70) MEASUREMENTS AFTER PRESS-CURING -Continued INITIAL MEASUREMENTS Capacitance (u F) Dissipation Factor ("/1) These results show a substantial improvement in breakdown voltage over the un-impregnated units in Example 1. Also an actual increase in capacitance is experienced.

EXAMPLE 111 This example is presented as definite evidence that capacitance is not decreased as a result of inclusion of bonding material between the dielectric film and the foil electrodes. In this test two simple three-plate stacked capacitors were made by positioning two nickel foil electrodes 0.001 inch thick by 1.25 inches square side by side about one-half inch apart. Over and under these were positioned two long nickel foils, 0.001 thick X /9 inch wide X 4 inches long. Connected together at one end these became common electrodes for both capacitor assemblies. Dielectric films of 0.003 inch polyimide film separated all the foils. With insulating film on either side of the assembly it was placed between platens to which incremental pressure increases were made by an air press with capacitance measurements made at each pressure at room temperature. The crossed plate area of each capacitor was about threefourths sq. inch and the pressures reported below are in pounds per square inch.

Pressure Capacitance of No. l Capacitance of No. 2

(psi) (picofarads) (picofarads) Pressure Capacitance of No. l Capacitance of N0. 2

(psi) (picol'arads) (picofarads) These results show that practically all excess resin is removed from between the foils and films at 132 psi and some increase in capacitance is obtained when this bonding agent is used over that of the dry pressed assem blies.-

' EXAMPLE iv e The same testwas made as in Example "I except a I liquid silicone casting and potting resin (Dow Corning Corp. Sylgardl82) was used. Capacitance measurements at various pressures Pressure Capacitance of No. l Capacitance of-No. 2

(psi) (picofarads) (picofarads) Capacitance measurements at various. pressures with silicone between all foils and films.

Pressure Capacitance of No. l Capacitance of No. 2

(psi) (picofarads) (picofarads) binder layer on one side of the polymeric dielectric means with first electrode means and contacting the adhesive binder layer on another side of the polymeric dielectric means with second electrode means to offset edges of the first electrode means fromadjacent edges of the second electrode means and to separate each of the electrode means in spaced relationship, moving the each of the electrode means to the polymeric dielectric of the first and second electrode means; removing adhesive binder from the offset edgesof thefirst electrode means to expose such edges and'removing adhesive binder from the offset edges of the second electrode means to expose such edges at an end of the capacitor opposite the end with exposed edges of the first electrode means, the polymericdielectric meansand the adhesive binder cooperatingto provide means sepfirst and second electrode means and the polymeric di- I electric-means intointimate engagement to adhere 'arating the first and second electrode means capable of second electrode means.

2. The method of claim 1, including the further steps of contacting adhesive binder-of a second polymeric dielectric means to a side of the first electrode means opposite the side of the first electrode means previously contacted by adhesive binder, and contacting adhesive binder of a third polymeric dielectric means to a side of the second electrode means opposite the side of the second electrode means previously contacted by binder means prior to the step of moving first and second electrode means and the polymeric dielectric means into intimate engagement. i

3. The method of claim 1, wherein the adhesive binder means is a polymeric binder means.

4. The method of claim 1, wherein the step of moving the electrode means into intimate engagement with the polymeric dielectric means includes the steps of pressing the electrode means and the polymeric dielectric means, and curing the adhesive binder.

,5. The method of claim 2, including the further steps of winding the electrode means and the polymeric dielectric r'neans into a convolutely wound body prior to heterocyclic linkages.

7. The method of claim 1, wherein each of the electrode means and the polymeric dielectric means are substantially flat.

UNETED tfiTATES PATENT OFFICE CERTIFICATE OF CORRECTION t Attlt-u 3, 51,3 3

DATE 1 i it %s csztifiec? that error appears in the above-identified patent and that said Letters Patent 5 are hen mrrected shown be uwi 3 Col. 2, line 37 Delete "aRe" and substitute therefore are-- i Col. 4, line 12 Delete "wellknown" and substitute therefore -wellknown Col. 6; line 45 Delete "wellknown" and substitute therefore --well--knowng Col. 8, line 49 Delete "MOst" and substitute therefore Most Col. 16, line 52 Delete line 52 5 Col. 17, line 46 Delete "siloxanephenolics" and substitute 1 therefore siloxane-phenolics Cols l7 and 18, first formula, insert CH before "2", first occurrence.

UNITED STATES PATENT OFFICE Page 2 CERTIFICATE OF CORRECTION Patent No. 3,851 $3 3 Dated December 5! 197a Inventor(s) James M. Rooe It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. .9, line 64 Delete "polyazeleicpolyanhydride" and substitute therefore -'-polyazeleicpolyanhydride--- Col. 25, line 9 Delete "2" and substitute therefore -l-- Signed and sealed this 24th day of June 197.5.

(SEAL) Attest:

C. IIARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

1. A METHOD OF MAKING A CAPACITOR INCLUDING THE STEPS OF PROVIDING POLYMERIC DIELECTRIC MEANS HAVING OPPOSITE SIDES, EACH OF THE OPPOSITE SIDES HAVING A LAYER ON ONE SIDE OF THEREON, CONTRACTING THE ADHESIVE BINDER LAYER ON ONE SIDE OF THE POLYMERIC DIELECTRIC MEANS WITH FIRST ELECTRODE MEANS AND CONTACTING THE ADHESIVE BINDER LAYER ON ANOTHER SIDE OF THE POLYMERIC DIELECTRIC MEANS WITH SECOND ELECTRODE MEANS TO OFFSET EDGES OF THE FIRST ELECTRODE MEANS FROM ADJACENT EDGES OF THE SECOND ELECTRODE MEANS AND TO SEPARATE EACH OF THE ELECTRODE MEANS IN SPACED RELATIONSHIP, MOVING THE FIRST AND SECOND ELECTODE MEANS AND THE POLYMERIC DIELECTRIC MEANS INTO INTIMATE ENGAGEMENT TO ADHERE EACH OF THE ELECTRODE MEANS TO THE POLYMERIC DIELECTRIC MEANS AND TO MOVE ADHESIVE BINDER FROM BETWEEN THE POLYMERIC DIELECTRIC MEANS AND THE ELECTRODE MEANS INTO THE AREA OF THE ADJACENT OFFSET EDGES TO COVER EDGES OF THE FIRST AND SECOND ELECTRODE MEANS, REMOVING ADHESIVE BINDER FROM THE OFFSET EDGES OF THE FIRST ELECTRODE MEANS TO EXPOSE SUCH EDGES AND REMOVING ADHESIVE BINDER FROM THE OFFSET EDGES OF THE SECOND ELECTRODE MEANS TO EXPOSE SUCH EDGES AT AN END OF THE CAPACITOR OPPOSITE THE END WITH EXPOSED EDGES OF THE FIRST ELECTRODE MEANS, THE POLYMERIC DIELECTRIC MEANS AND THE ADHESIVE BINDER COOPERATING TO PROVIDE MEANS SEPARATING THE FIRST AND SECOND ELECTRODE MEANS CAPABLE OF WITHSTANDING ABOUT 3,000 VOLTS OR MORE PER MIL BETWEEN THE FIRST AND SECOND ELECTRODE MEANS, AND ATTACHING TERMINAL MEANS TO EXPOSED EDGES OF THE FIRST AND SECOND ELECTRODE MEANS.
 2. The method of claim 1, including the further steps of contacting adhesive binder of a second polymeric dielectric means to a side of the first electrode means opposite the side of the first electrode means previously contacted by adhesive binder, and contacting adhesive binder of a third polymeric dielectric means to a side of the second electrode means opposite the side of the second electrode means previously contacted by binder means prior to the step of moving first and second electrode means and the polymeric dielectric means into intimate engagement.
 3. The method of claim 1, wherein the adhesive binder means is a polymeric binder means.
 4. The method of claim 1, wherein the step of moving the electrode means into intimate engagement with the polymeric dielectric means includes the steps of pressing the electrode means and the polymeric dielectric means, and curing the adhesive binder.
 5. The method of claim 2, including the further steps of winding the electrode means and the polymeric dielectric means into a convolutely wound body prior to the step of attaching terminal means to exposed edges of the first and second electrode means.
 6. The method of claim 3, wherein the polymeric dielectric means is a material selected from olefin polymers, condensation polymers, and polymers containing heterocyclic linkages.
 7. The method of claim 1, wherein each of the electrode means and the polymeric dielectric means are substantially flat. 